The present invention relates to a method using a single pick-up head to read and store data in discs of different thickness, especially for CD (compact disc) and DVD (digital versatile disc), the including the step of adjusting the electronic aperture ring to change the numerical aperture, and change the distance between the disc and the object lens subject to type of the disc used. This invention relates also to the pick-up head for the application of the method.
Regular compact disc drivers are designed subject to the specifications of the laser discs. Regular compact discs include music compact discs and CD-ROMs. Since the establishment of the world DVD (digital versatile disc) standard, the pick-up head has to provide two focusing points to read CD and DVD, following is a comparison table between a CD and a DVD:
______________________________________ THICKNESS TRACK PITCH WAVELENGTH mm .mu.m nm NA ______________________________________ CD 1.2 1.6 780 0.4-0.45 DVD 0.6*2 0.74 635- -650 0.6 ______________________________________ NA:Numerical Aperture.
CD drivers and DVD drivers use laser light of different wavelength to read data from CD and DVD respectively. Because the thickness of a CD is different from that of a DVD, a common pick-up head cannot be used to read both CD and DVD. Because of the requirement for compatibility, a DVD driver must be designed to read discs of different thickness. There are several methods which achieve this requirement. These methods include; 1) using two object lenses to read discs of different thickness; 2) using a HOE (holographic optical element) to provide two focusing points; and 3) using a LCD shutter. However, using two object lenses to read discs of different thickness greatly increases the weight of the pick-up head and its manufacturing cost, when a HOE is used, the light output of the laser diode will be splitted into several different orders, thereby causing the noise level to be relatively increased and the emission light power of laser diode has to increase too. Furthermore, HOE is difficult and expensive, to manufacture. If a LCD shutter is used, the polarization of the laser light source must be properly arranged relative to the LCD and polarizer. Therefore, this design is difficult to install. Furthermore, the LCD need continous power supply to maintain shutter effect.